Chapte 19 Part 4: Generating the Dorsal Ventral Axis

Question 1

D-V polarity of the limb bud is determined by the ___- encasing it

Answer 1

ectoderm

Question 2

Wnt7a is expressed in the ___ ectoderm of the chick and mouse. Wnt7a then induces activation of ____ in the dorsal _____. What does this do?

Answer 2

Wnt7a is expressed in the DORSAL ectoderm of the chick and mouse. Wnt7a then induces activation of LIM1 in the dorsal MESENCHYME. Lim1 gene inscirbes a transcription factor responsible for dorsal phenotype determination (ie/ nails and top of hand).

Question 3

What happens if Wnt7a in the ectoderm is overexpressed?

Answer 3

Overexpression of Wnt7a causing outgrowth of AER and extra digits (affects proximal-distal patterning). WNT7A ALSO ACTIVATES SHH (along with Fgf4 from aer), most likely contributing to proliferation and outgrowth.

Question 4

___ marks ventral ectoderm of the limb bud, and is induced by ___ in the underlying mesoderm.

Answer 4

ENGRAILED marks ventral ectoderm of the limb bud, and is induced by BMPs in the underlying mesoderm

Question 5

What would happen if BMPS were knocked out in the early limb bud?

Answer 5

Engrailed1 would not be epressed, and wnt7a would be expressed even in the ventral side of the limb bud. This would prevent the formation of footpads/ palm

Question 6

What is the ventral side of the limb

Answer 6

palm side

Question 7

How is BMP involved in the complete end of limb patterning?

Answer 7

Recal: when fgfs in AER get too high, they suppress Gremlin (a BMP inhibitor), thus, BMP is now active. BMP inhibits the AER. Without Fgfs and Wnts being produced to sustain the ZPA, SHH stops being released and ZPA shuts down. Wnt7 in ectoderm is now stopped, and the anterior posterior axis is now stopped, resulting in all three axes to stop developing.

Question 8

FORMATION of digits and joints

What are the cells between digit cartilage that is programmed to die

Answer 8

the interdigit necrotic zone.

Question 9

FORMATION of digits and joints

the interior necrotic zone separates the ___ and ___

Answer 9

separates the ulna and radius.

Question 10

FORMATION of digits and joints

the anterior and posterior necrotic zones separate the:

Answer 10

ends of the limbs.

Question 11

FORMATION of digits and joints

Cells of the necrotic zones die via _____ following fragmentation of ______.

Answer 11

die via apoptosis following fragmentation of their DNA

Question 12

FORMATION of digits and joints

____ in the mesenchyme induce apoptosis, but is dependent on the upregulated synthesis of interdigital ____

Answer 12

BMPS in the interdigit mesenchyme induce apoptosis, but is dependent on the upregulated synthesis of interdigital RA.

BMPS are also uninhibited because SHH in the random mesencyme not ZPA inhibits Gremlin to upregulate BMP, resulting in cartilage formation.

Question 13

Which BMPS are found in the interdigit mesenchyme

Answer 13

BMPs 2,4, and 7.

Question 14

Digit 1 is not exposed to SHH during SPECIFICATION (not formation), resulting in the specification of the thumb. How is this Digit then specified?

Answer 14

digit 1 is not expoesd to SHH and is instead specified by Hoxd13 in the absense of Hoxd12. Forced expression of Hox13 results in the formation of the thumb.

Question 15

BMPS in the interdigit mesenchyme specify and help form the digits ___ to it

Answer 15

anterior to it.

Question 16

If BMPS in the interdigit mesenchyme induce apoptosis of the interdigit webbing, how does the cartilage in the actual digits not get killed off?

Answer 16

the cartilage cells secrete Noggin, preventing apoptosis in the limb bones.

Question 17

What is context dependency

Answer 17

when a factor can elicit opposing effects depedning on the history of the cell.

Question 18

explain context dependency in the limb joint

Answer 18

In early limb bud formation, BMP2 and BMP7 induce apoptosis in the interdigit mesenchyme (necrotic zone)

In the later limb stages: BMP2 and BMP7 are required in order for bud cells to form cartilage

The function first ascribed to BMPs was the formation, not the destruction, of bone
and cartilage tissue.

Question 19

explain context dependency in the limb joint

Answer 19

In early limb bud formation, BMP2 and BMP7 induce apoptosis in the interdigit mesenchyme (necrotic zone)

In the later limb stages: BMP2 and BMP7 are required in order for bud cells to form cartilage

The function first ascribed to BMPs was the formation, not the destruction, of bone and cartilage tissue. Now this is reversed

Question 20

Joint formation

Given the context-dependent nature of this process, when do BMPS induce apoptosis rather than forming cartilage?

Answer 20

When FGFs from the AER are present, Dkk is activated. This protein mediates apoptosis and at the same time inhibits Wnt from aiding in skeleton formation.

Question 21

Several BMPs are made in the ______ cells surrounding the condensing chondrocytes and promote further cartilage formation. Another BMP, ____, is
expressed at the regions ____ the bones, where joints will form, and appears critical for joint formation

Answer 21

Several BMPs are made in the perichondrial cells surrounding the condensing chondrocytes and promote further cartilage formation. Another BMP, Gdf5, is
expressed at the regions between the bones, where joints will form, and appears critical for joint formation

Question 22

How do Wnts help with joint formation?

Answer 22

Wnt proteins are critical in sustaining transcription of Gdf5 (BMP FOUND IN REGION OF JOINT FORMATION), and β-catenin produced by the Wnts is able to suppress the Sox9 and collagen-2 genes that characterize precartilage cells

You can’t have precartilage cells in the area of joint formation. This allows the bone of this area of the limb to be eroded away and to have joints form in this area.

Question 23

How do Wnts help with joint formation?

Answer 23

Wnt proteins are critical in sustaining transcription of Gdf5 (BMP FOUND IN REGION OF JOINT FORMATION), and β-catenin produced by the Wnts is able to suppress the Sox9 and collagen-2 genes that characterize precartilage cells

You can’t have precartilage cells in the area of joint formation. This allows the bone of this area of the limb to be eroded away and to have joints form in this area.

Question 24

Wnt proteins are critical in sustaining transcription of ____ (BMP FOUND IN REGION OF JOINT FORMATION), and β-catenin produced by the Wnts is able to suppress the _____ and ____ genes that characterize precartilage cells

Answer 24

Wnt proteins are critical in sustaining transcription of Gdf5 (BMP FOUND IN REGION OF JOINT FORMATION), and β-catenin produced by the Wnts is able to suppress the Sox9 and collagen-2 genes that characterize precartilage cells

Question 25

In normal joint formation, the cells that form the joint will lose their chondrocyte characteristics (because Wnts (4 and 9) in the area promote GDNF expression, and repress Soz9 and Collagen2 genes that usually make chondrocytes). What other protein is necessary for normal joint formation?

Answer 25

Ext1, a protein that is also expressed by the cells losing their chonrdrocyte characteristics that form the joint. Ext1 is a protein necessayry for synthesizing heparan sulfate.

Question 26

In normal joint formation, the cells that form the joint will lose their chondrocyte characteristics. What do these cells express? What exactly do these cells form?

Answer 26

Gdf5 (type of BMP), Wnt4 and 9 (which help sustain GDNF expression, and repress Soz9 and Collagen2 genes that usually make chondrocytes), and Ext1 (for heparan sulfate formation)

These cells will form the articulate cartilage (joint cartilage rather than bone cartilage) and the synovium, which secretes lubricating fluid.

Question 27

Cells expressing GDNF, Wnt 4, Wnt 9 and Etv1 in the bone will form the ____ cartilage (joint cartilage rather than bone cartilage) and the ____, which secretes lubricating fluid.

Answer 27

These cells will form the articulate cartilage (joint cartilage rather than bone cartilage) and the synovium, which secretes lubricating fluid.

These cells thus form the joints between the bones.

Question 28

Why are some limbs webbed?

Answer 28

Gremlin is an inhibitor of BMP. Some animals prevent BMP from being able to act as an apoptosis-inducer in the interdigit mesenchyme, resulting in webbing

Gremlin is expressed in duck hindlimb, which prevents cells from undergoing apoptosis, but in chick hind limbs, there is no gremlin, allowing BMP to function and cause apoptosis in the interdigit mesenchyme